[clang] [Clang] Don't diagnose VLA for `-std=gnu++*` by default (PR #89943)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman requested changes to this pull request.

This was discussed in this RFC: https://discourse.llvm.org/t/rfc-diagnosing-use-of-vlas-in-c/73109 and our decision to move forward with that is independent of what GCC does. Without more justification for walking this back, I do not agree with moving forward with the revert.

https://github.com/llvm/llvm-project/pull/89943